1. Field of the Invention
The present invention relates to a ceramic filter that collects particulate matter included in the exhaust gas emitted from automobile engines, and a catalyst-loaded ceramic filter that has a catalytic function to remove the particulate matter.
2. Description of the Related Art
It has been a practice to collect fine particles such as soot (particulate matter) included in the exhaust gas emitted from diesel engines, by means of a particulate collecting filter (DPF) installed in the exhaust gas passage. The DPF employs a honeycomb structural body made of porous ceramic material as a filter substrate, that has a large number of cells formed parallel to the direction of the exhaust gas flow. Cells that adjoin each other are separated by a porous wall, with one end of each cell being stopped, at the inlet or outlet side in a staggered arrangement, so that the particulate matter is collected while the exhaust gas flows through the porous walls between the cells.
The particulate matter that has been collected can be removed by burning, with for example, an electric heater or a burner, periodically. Unfortunately, this method has a drawback in that the DPF is heated excessively during burning when too much particulate matter has been collected, eventually leading to breakage of the DPF due to thermal stress. To prevent this problem, a catalyst-loaded DPF has been proposed that burns the particulate matter through a catalytic reaction in order to lower the burning temperature. Various methods have been studied which would the catalyst-loaded DPF to burn and remove the particulate matter, such as direct oxidation of the particulate matter, or oxidizing NO included in the exhaust gas into NO2, and oxidizing the particulate matter with NO2.
However, the DPF materials (cordierite, SiC, etc.) currently in use are not capable of directly supporting a catalyst metal and, accordingly the catalyst-loaded DPF is usually wash-coated with γ-alumina or the like on the surface of the filter substrate so as to form a coating layer that supports the catalyst metal. However, this constitution has a decreased porosity in the filter substrate due to the formation of the coating layer, resulting in very high pressure loss (two to three times that of a filter substrate having a porosity of 50%, when the coating layer is formed) and a significant increase in the weight (two to three times that of a filter substrate having a porosity of 50%, when the coating layer is formed). There are also problems where the coating layer reduces the thermal shock resistance due to a higher coefficient of thermal expansion and impedes early development of catalytic activity due to an increased heat capacity.